An operational transconductance amplifier (OTA) typically receives differential input voltages and produces an output current. The OTA is similar to a standard operational amplifier in that the OTA has a high impedance differential input stage and may be used with negative feedback. The OTA may be implemented in various circuits. For example, the OTA may be used in a switched-capacitor circuit, such as a sample/hold circuit of pipelined analog-digital converters (ADCs).
FIGS. 1A and 1B show at 100 an example of a diagram of a flip-around sample/hold circuit of ADCs and an example of timing diagrams of switches in the diagram of the sample/hold circuit. As shown in FIG. 1A, the sample/hold circuit includes a differential input 102, input switches 104, an operational transconductance amplifier (OTA) 106, sample/hold capacitors 108, reference terminals 110, reference switches 112, loading capacitors 114, and feedback switches 116.
Differential input voltage signals may be provided at the differential input 102. The input switches 104 control when to pass the differential input voltage signals to input terminals of the OTA 106. A direct current (DC) common-mode voltage signal, VCM, is supplied to the reference terminals 110 to provide proper DC points for the OTA 106. The OTA 106 may generate two output voltage signals to be applied to the loading capacitors 114. The feedback switches 116 control two feedback loops of the OTA 106.
The timing diagrams of the reference switches 112, the input switches 104, and the feedback switches 116 are shown at 118, 120, and 122 in FIG. 1B, respectively. During the sampling phase, both the input switches 104 and the reference switches 112 are on (at a logic high level) and the feedback switches 116 are off (at a logic low level). The right-side plates of the sample/hold capacitors 108 that are connected to the input terminals of the OTA 106 are set to VCM through the reference switches 112. The left-side plates of the sample/hold capacitors 108 that are connected to the differential input 102 through the input switches 104 are set to follow the differential input voltage signals.
After a predetermined period of time, the reference switches 112 are turned off (at a logic low level). The values of the differential input signals are stored on the sample/hold capacitors 108. The input switches 104 are turned off slightly later than the reference switches 112 so that charge injection and sampling time may be signal-independent. The left-side plates of the sample/hold capacitors 108 are disconnected from the differential input 102. After another predetermined period of time, the feedback switches 116 are turned on (at a logic high level). The left-side plates of the sample/hold capacitors 108 are connected to output terminals of the OTA 106 through the feedback switches 116. This is called “flip-around,” where the left-side plates of the sample/hold capacitors 108 are “flipped” from the differential input 102 to the output terminals of the OTA 106. The values of the differential input voltage signals stored on the sample/hold capacitors 108 are held at the output terminals of the OTA 106 and passed to the loading capacitors 114.